The present invention relates to a method for generating frequencies in a direct conversion transceiver of a radio communication system operating in two different frequency bands, wherein a first frequency band comprises a first transmission frequency band and a first receiving frequency band, and a second frequency band comprises a second transmission frequency band and a second receiving frequency band. The invention also relates to a direct conversion transceiver of a radio communication system operating in two different frequency bands. In addition, the invention relates to the use of the method and the direct conversion transceiver in a mobile station.
It is a known method to use a reference oscillator and one or more frequency synthesizers to generate local oscillator frequencies for the transmitter and receiver. For practical reasons, the frequency of the reference oscillator is considerably lower than the local oscillator frequencies. As is generally known and as shown in FIG. 2, the frequency synthesizer consists of a phase locked loop (PLL), by which the output frequency of the VCO is locked at the reference frequency. In this loop, the reference frequency and the frequency of the voltage controlled oscillator (VCO) are taken as divided to a phase comparator, the filtered output voltage of which is the control voltage of the voltage controlled oscillator VCO. The control voltage controls the oscillator in a manner such that its frequency is locked at the frequency coming from the reference frequency branch to the phase comparator.
In a direct conversion receiver, the local oscillator frequency is generated at the received channel frequency, whereby the received baseband signal is obtained directly as the difference between the received radio frequency signal and the local oscillator frequency. Similarly, in a direct conversion transmitter the local oscillator frequency is tuned at the transmission channel, whereby in addition to the local oscillator frequency, the modulating signal is also directed to the mixer. The mixing result includes a modulated signal at the transmission frequency, which is directed through the normal filter and amplifier stages to the antenna. A direct conversion transceiver is of a simple construction; particularly the number of radio frequency blocks is smaller than in the ordinary transceivers which include intermediate frequency stages.
In a direct conversion transceiver, the local oscillator frequency is at the actual transmission or reception frequency. If full duplex operation is not required in the system, it is possible to implement the transceiver using only one frequency synthesizer. However, a problem is, that the tuning range of the frequency synthesizer can become very wide, because it must cover both the transmission and reception channel frequencies. Furthermore, the frequency synthesizer must be capable of removing the duplex offset frequency when switching from reception to transmission or vice versa (Duplex Separation). Implementing a dual band direct conversion transceiver, such as a GSM/PCN mobile station, with the solutions known at present practically always requires the use of at least two frequency synthesizers. In some cases as much as four separate frequency synthesizers are needed, that is, separate frequency synthesizers for both the receiver and transmitter and for both frequency bands, because the tuning range required may become too wide when only one frequency synthesizer for each frequency band is used.
In the dual band transceiver shown in FIG. 1, separate frequency synthesizers for each frequency band are used in the radio frequency part. In a mobile station, such as GSM/PCN, this means that the frequency synthesizer used in the GSM mode of operation operates in the frequency band 880 to 960 MHz, whereas the frequency synthesizer used in the PCN mode of operation operates in the frequency band 1710 to 1880 MHz.
Since the PCN frequency band is about twice the GSM frequency band, it might be possible to implement a solution with only one frequency synthesizer if the PCN frequencies were generated by multiplying the frequency generated by the frequency synthesizer by two. In this case, the tuning range of the frequency synthesizer would be 855 to 940 MHz in the PCN mode of operation. Thus the tuning range of the frequency synthesizer should be 855 to 960 MHz, which is still relatively high, approximately 11.6%. The frequency offset that the frequency synthesizer should be capable of 45 MHz (Duplex Separation) is also relatively high.
One more possible implementation of the prior art solutions is to mix the frequency of the frequency synthesizer either on the transmission or reception side with a fixed frequency, 45 MHz in a GSM/PCN receiver, in order to remove the frequency offset between transmission and reception. Thus the tuning range of the frequency synthesizer can be decreased. However, in order to generate a fixed frequency, the oscillator must have a very stable frequency and it must be capable of being locked at an exact reference. This oscillator can be implemented by means of frequency synthesis, for example. The practical implementations thus resort to the use of two frequency synthesizers.
In practice, due to the requirements concerning speed, noise etc., the maximum tuning range with the frequency synthesizer is approx. 10% of the nominal frequency. The speed and noise requirements are opposed, that is, the faster change of frequency is desired, the higher is the noise of the frequency synthesizer, and, respectively, the less noise is desired, the slower is the change of frequency. The speed requirement, that is, the time allowed to move from one frequency of the frequency synthesizer to another varies between radio systems, being 600 to 800 microseconds in GSM and PCN, for example.